1. Field of the Invention
The invention is directed to an electrically heated toaster with at least one bread holder which is maintained in toasting position by an electromagnet, which can be tripped manually as well as electrically by a timer, which includes a clock pulse controlled counting stage, which is in circuit connection with a frequency determining charging capacitor, which can be charged through a potentiometer for presetting the degree of toasting, as well as being in circuit connection with a voltage divider, which includes a temperature-dependent resistance for compensation of the influence of the toasting area temperature, with the counting stage triggering the electromagnetic tripping device when the preset degree of toasting has been reached.
2. Description of the Related Art
A toaster with such a tripping device has become known from the German patent publication 30 20 779 C2. The bread holder is put into a toasting position by a switching lever counter to the force of a spring and it is locked in this position by an electromagnetic pawl. At the same time the heating resistances and an electronic time control are switched on, which, after the preset degree of toasting has been reached, operates a switch in the circuit with due regard to the temperature in the toasting space, in order to release the latch or lock between the bread holder which has been lowered into its operating position, and the chassis. If necessary, this lock or latch can also be released manually. After the latch has been released the current supply of the time control and the heating coil is also switched off.
This combined known tripping device, which can be actuated directly by hand through a switching or actuating lever as well as by an electromagnet through an adjustable timer, has disadvantages as far as its electronics as well as the mechanical structure are concerned.
A comparator is provided in the control circuit, at one of whose inputs a series circuit consisting of a potentiometer for presetting the degree of toasting and a frequency determining charging condenser is connected and at whose other input a voltage divider is connected, which includes a temperature-dependent resistance which is affected by the temperature in the toasting space. A counting pulse is given to a counting stage when the voltages at both inputs to the comparator are equal. The counting stage counts up to a specific sum, stretches the counting pulses in between and operates a switch in the current circuit of the electromagnet.
A simple unijunction transistor is to be used as a comparator. However, the known comparator circuit cannot be realized with this alone. Rather a programmed unijunction transistor has to be utilized, which is sensitive to the temperature fluctuations which can never be completely avoided in a toaster, the more so since the condition has to be observed, that only equal input voltages at the inputs are to trigger a counting pulse.
The feed voltage for the known time control is supplied by a rectifier, which is connected through a series resistance directly to the electric mains thus causing considerable losses. Apart from the unnecessary power loss in the series resistance measures for compensating voltage fluctuations must be taken.
In addition, a rapid and complete discharge of the frequency determining capacitor after its having emitted a counting pulse is not assured in the known control circuit. An inaccurately meaning not completely discharged charging condenser leads however necessarily to frequency oscillations and thus to an arbitrary triggering of the electromagnet.
An additional reset switch must be provided in order to reset the counting stage in the known timer.
All in all the known electronic timing control does not assure the precision expected from a domestic appliance in the long run, the more so since one operates with a pulse generation stage in connection with a counting stage. It must be added that the known triggering device includes an electromagnetic pawl, which is released by the electromagnet. In case of current failure the bread carrier thus remains in its latched position and the mains switch cannot be opened.